AWWA MTC61200

Since 1978, the City of Marysville has relied upon low-tech screening and free chlorine disinfection to supply customers with high-quality drinking water from the 3.24-mgd Stillaguamish River Ranney Collector groundwater source. Fluctuating turbidity forces the City to shut down the source when turbidity exceeds 1.0 NTU, which frequently results from wet weather conditions in the fall, winter and spring, but can occur at any time. In March 2000, the Washington Department of Health reclassified this source as groundwater under the influence of surface water. Faced with the Surface Water Treatment Rule (SWTR), the City elected to install filtration to bring this source into compliance. The City preliminarily evaluated eight filtration technologies against various regulatory, operational, and cost criteria. Of those technologies, membrane filtration appeared to best meet the criteria for this source because it provides effective treatment, it accommodates variable production, it meets future regulations, the reject flow is relatively low, and the capital and O&M costs are minimized by the small footprint and low level of operator attention required. Having selected membrane filtration, the City compared a microfiltration (MF) pressure-driven membrane and an ultrafiltration (UF) submerged membrane during a 6-month pilot study. The pilot study revealed that both membranes produced exceptional filtered water quality and performed better than expected in treatment of this source. In general, the UF membrane net recovery proved slightly higher, the UF filter run recovered and was lengthened following a raw water turbidity spike, the UF membrane required fewer chemical cleanings and required less hazardous cleaning chemicals, and the UF membrane projected annual O&M costs were lower. The City selected the UF membrane for full-scale design. Because the membranes are packed efficiently in the membrane tank and the remaining equipment is typically located adjacent to the membrane tanks, only a relatively small building is required. The entire plant can be housed on a small, 1.3-acre site, providing substantial savings in property acquisition costs. The City also realized design savings by entering into a pre-purchase agreement with the UF supplier to provide all associated membrane equipment, including the membrane tanks, pumps, flow meters, chemical feeding equipment, and all programming for their equipment. The remainder of the design consists of locating the equipment in a building, designing the connecting piping, site/civil design, and designing other ancillary facilities or processes. Includes figures.